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Author Topic: Ants; which way of thinking?  (Read 270 times)
KD7EVS
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« on: July 07, 2003, 12:43:44 AM »

So I'm building and testing different parts and configurations for 1/4 wave 2m antennas. for future knowledge mostly.

ofcourse I've built the standard  3 and 4 radial antenna's out of the so239, as well as the drooping radial in both 3 and four radial models.  

I've been testing everything hanging on a non-conductive string, from an non-conductive arm. there are no objects within 15-20 horizontal feet. connected to it is coax running to my VX-1r running one watt from an external gel cell. testing it in QSO with the xyl simplex from work (4 city miles) and into mountain repeaters 70-80+ miles away.    

all that being said; my best one was the four radials downtilted at around 45ish degrees. (sorry antenna's aren't an exact science ...when not in free space).

I've now worked on and am testing a new one I made. I seem to be able to work all the same places and then some. Add a little bit of range on simplex. Oh and I can qso on a egopeater I couldn't touch before. it seems to hear better on simplex than before as well.

With this added distance has come some tradeoff. I now get pager interference on my several 2m freqs. (probably most, but at least the three i hang out on).
I haven't had a problem with this on the other ones. now, I know my whip is the length it's supposed to be, the ground system is what I'm really testing.   either A; today the pager systems switch freqs right before I hooked up my new antenna. or B; this new antenna is great at picking it up. oh, and there is no interference on tx signal.

so my thoughts are... this antenna design might be too broadband? the extra gain (directivity) might be bringing in the signal enough to break squelch and I just didn't notice it earlier?  

my mobile setup doesn't pick this up and it has a 4.5db gain 7foot 2m antenna on it. I know any 2m mobile is going to have better filtering than the broadband vx-1r. I'm going to do tests tomorrow (noone is paging right now) with a B/King 2-5 watt HT. that should show me how much the radio is dictating the reception of interference.

thoughts on this situation? since it's most likly only a problem on the poorly filtered vx-1r am I hearing this because the antenna is working, or because this design is too broadband or both?

my intentions for this is emergency deployment operations, ATV mobile, etc..  something bullet proof, portable and something that works.

done rambling now. been up too long, going to sleep.
seven three
KD7EVS

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IM4UKWIDCATS
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« Reply #1 on: July 07, 2003, 01:12:30 AM »

well, a 1/4 ground plane antenna should have the radials with a 45 degree down tilt - this is the way to pull the antenna input to close to 50 ohms.

The intermod is a different problem.

Yes, if you put a higher gain antenna on a handheld radio, you will pick up paging intermod if it exists. This is not that the antenna is too wide/broadbanded, it is that the poor little hand held is being eaten by the intermod. If you have the same intermod problem on the Bendix King, I would be VERY surprised. The same holds true for the mobile, while many wide band mobile rigs are not as good as a monoband radio with a helical frontend, it is better than the hand held.



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K0IPG
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« Reply #2 on: July 07, 2003, 08:53:06 AM »

If all these antennas are just 1/4-waves with different ground systems (I believe that's what you said), then you shouldn't be seeing that much difference - you must be changing something else in the system.  The difference between 3 and 4 ground radials isn't going to make a repeater you couldn't hear before magically become usable - something else must have changed. And perhaps that "something" is as simple as the time of day. If you've been spreading your tests out over many hours, the changes you've noticed could have been due to a change in atmospheric conditions or something like that.

If you really want to experiment with different 2m antennas, try something besides a 1/4-wave. A jpole is easy to constuct, and I think you'll be pleasantly surprised at it's performance over your 1/4-wave!

73
Dan K0IPG
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KD7EVS
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« Reply #3 on: July 07, 2003, 06:11:49 PM »

yes all the antenna's are quarter wave, ground planes. but that's the only similarity. I've tried countless different systems consisting of different materials and in different configurations. some have preformed while others haven't.  

""""" If you really want to experiment with different 2m antennas, try something besides a 1/4-wave. A jpole is easy to constuct, and I think you'll be pleasantly surprised at it's performance over your 1/4-wave! """""

I didn't say I want to experiment with antennas, I said I was experimenting with 1/4wave antennas. as for J-poles... I don't want a j-pole, I've built too many of those and they do not suit this situation. if I wanted something that "really" worked I'd throw together another colinear. but alas, I'm on Qwaves.

as I said very specificly I am testing quarter wave systems. there are very specific design constraints in this application. mainly there can be no part of the antenna that extends more than 21 inches from the feedpoint, i have to be able to take is apart, do field repair (no breakable joints) and no sharp points (i.e. radials) other than the whip straight up.

so this is much more complex than just picking between three or four radials.
as for time of day, it's been over the last couple weeks. but I keep going back to my standard 4 radial system as a 'control antenna.' if it works worse, I toss it. if if does better I keep if for a while to continue testing.

so the other ham basicly agreed with what I was thinking. most antennas with enough gain should pick up itermod (if there) easier.   now to go hook vx-1r up to my colinear mobile antenna to see if that one picks it up.

seven three.
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N6AJR
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« Reply #4 on: July 07, 2003, 08:04:04 PM »

Remember. a duck is a lousy antenna. the quarter wave with radials is a much better antenna. You are transmitting and recieving more power so local rf will affect you much more.  Put up a 20 element beam and listen tho the extraneous garbage you hear on the radio.  Higher gain is just that, you are hearing better and transmitting better.  arent antennas fun.
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K0IPG
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« Reply #5 on: July 07, 2003, 10:30:14 PM »

Well, if you're actually talking about just a 1/4-wave antenna, then I don't understand what it is that you're changing about it's design parameters. Your "whip" will always be exactly a 1/4-wave long on the desired operating frequency. Your radials should always be sloped downward at a 45 degree angle (this adjusts the impedance so it's closest to 50 ohms - if you have access to an antenna analyser, you could adjust them so that the antenna presents precisely 50 ohms, but it wouldn't be worth the trouble). And 4 radials are typically preferred over 3.

So.. if you don't mind me asking - what is it that you're changing about these antennas?

Quite frankly, nothing you do to a standard marconi antenna (referenced to your radial system rather than the ground) is going to make a noticable difference in performance, especially on 2m FM. Any changes that you've noticed are more likely to be the result of changing atmostpheric conditions, or changes that you've induced yourself with your test setup. For example : you mentioned that you test them by calling your XYL at work (4 mis away). I'm assuming (therefor, I could be wrong...) that your XYL is probably using a handheld. How do you know that for antenna test 1, she was holding the HT with the antenna at the same angle as it was during antenna test 2? If she had held it with the antenna "sloping" in relation to the ground, that would induce a loss measurable in your receiver.

I guess I just don't understand what it is that you're trying to "optomize" about this antenna's performance. It works the way it works, and you really can't do anything to improve on that.

If you are looking for an antenna that fits a tight space requirement, though, you still have other options (aside from marconis and jpoles, etc). Have you considered halos? There are a few options...

73
Dan K0IPG
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KD7EVS
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« Reply #6 on: July 07, 2003, 11:28:16 PM »



A) the xyl was/does use a mobile antenna (a colinear and mobile back down to 2watts). and just before you jump the gun; same parking spot.

"""" I guess I just don't understand what it is that you're trying to "optomize" about this antenna's performance. It works the way it works, and you really can't do anything to improve on that. """"

b) when have hams ever been happy with "it works the way it works" the day we get happy with that is the day I switch hobbies.

c) I know a dummy load er excuse me "rubber duck" doesn't pick up much of anything and therefore doesn't pickup intermod. I haven't been comparing this to a duck, I'm comparing this to my standard antenna which i mentioned.


d) I'm not looking to enhance the preformance. what I want is an antenna that meets the classifications that I set for it and works at least as well as the standard 4 radial (@ -45degrees). but I can't have pointy radials.

what I've just done, I believe, works better than this standard I have set, and meets all my requirements. I was just up after a long early morning shift at work and needed someone to back up my way of thinking. That is that reciving intermod on antenna B where I don't recieve it on antenna system A, where antenna(a or b) is the only variable that has changed in the setup. verified by the fact that simplex signal reports are better under the same conditions on antenna B.

I will tell you that the antenna that's working a bit better uses a solid, lightweight ground plane. the metal is round, and near the edges bends down to get somewhat closer to 50. it works, it meets the criteria.     fine for now, I'll pass my experimenting to something else.

73
zeb
 
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W0FM
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« Reply #7 on: July 09, 2003, 06:55:43 PM »

I suspect that it is the time of day of your comparisons that really makes the difference.  It's not the slope of the radials or the parking spot.

While a better antenna (anything better than the rubber duck) can overload the front end of a wide band receiver with paging intermod, two similar antennas should not be all that different.

Paging carriers have the same "rush hours" that commuters deal with.  Heavy first thing in the morning, at lunch,  and again at 5-6PM.  Practically nil on Saturday and Sunday compared to weekdays.

See how those times correlate to your experiments.

Good luck.

Terry, WØFM
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